1. Field of the Invention
The present disclosure relates to an apparatus for controlling an inverter, and more particularly to an apparatus for controlling an inverter by compensating for a slip frequency of a motor in a reference frequency of an inverter.
2. Description of the Related Art
A medium-voltage inverter is an inverter having a root-mean-square (RMS) input line-to-line voltage of 600V or higher. The medium-voltage inverter may have various rated power capacities ranging from hundreds of kilowatts (kW) to dozens of megawatts (MW), and has been widely used in various application fields, for example, fans, pumps, compressors, etc.
A representative example of the medium-voltage inverter is a multi-level inverter configured to output a voltage of at least 3 levels. Specifically, a cascaded H-bridge inverter has been widely used as the medium-voltage inverter. The amplitude of each output voltage level of the cascaded H-bridge inverter and the number of output voltage levels of the cascaded H-bridge inverter may be determined according to power cells contained in the cascaded H-bridge inverter, and an insulated voltage may be input to each power cell.
In the meantime, the induction motor widely used in industrial systems can be frequency-controlled by voltage/frequency (V/F) operation, and has been widely used in various application fields such as fans, pumps, blowers, etc. not requiring rapid operation characteristics in an operation region having a rated speed or less. However, a slip frequency may occur in the induction motor according to application load in which load is changed, such that it may be impossible to operate the induction motor at a given speed. Specifically, in a technical field such as a conveyor in which a given-speed operation is requested, there is a need for the slip frequency to be properly compensated in a manner that the actual operation speed is identical to a reference speed.
FIG. 1 is a view illustrating a conventional multi-level medium-voltage inverter system. FIG. 1 illustrates a cascaded H-bridge structure composed of a two-level unit power cell. The root-mean-square (RMS) line-to-line voltage of a main supply 200 may be a 3-phase voltage of 600V or higher. The output voltage of the inverter 100 may be input to a high-voltage 3-phase motor 300. The motor 300 may be an inductor motor or a synchronous motor.
A phase shift transformer 110 of the inverter 100 may insulate the main supply 200, and may change the phase and magnitude of a voltage according to a request of a unit power cell 120. In addition, total harmonic distortion (THD) of an input current of the main supply 200 may increase due to phase shifting.
The unit power cell 120 may use an output signal of the phase shift transformer 110 as an input signal, and output voltages of power cells of the respective phases are then synthesized, resulting in configuration of the output signal of the inverter 100. The a-phase output voltage of the inverter 100 may be a sum of output voltages of the unit power cells (120a1, 120a2) connected in series. Likewise, the b-phase output voltage may be a sum of unit power cells (120b1, 120b2) connected in series, and the c-phase output voltage may be a sum of unit power cells (120c1, 120c2) connected in series. In association with the output voltage of the synthesized medium-voltage inverter 100, the respective phase voltages may have a phase difference of 120 degrees therebetween whereas they have the same amplitude. In addition, with the increasing number of unit power cells 120 contained in the medium voltage inverter 100 and various switching actions, a total harmonic distortion (THD) and voltage regulation rate (dv/dt) of the output voltage applied to the motor 300 can be improved.
FIG. 2 is an example of the unit power cell illustrated in FIG. 1, and the unit power cell of FIG. 1 may be modified into various shapes.
Referring to FIG. 2, a rectifying unit 121 may convert an AC voltage received from the phase shift transformer into a DC voltage, and may include a plurality of diodes. The amplitude of the rectified DC-link voltage may be decided by the relationship of a difference between input power of the rectifying unit 121 and output power of an inverter unit 123. If the input power received from the rectifying unit 121 is higher than output power consumed by load, the DC-link voltage may increase. If the input power received from the rectifying unit 121 is equal to or lower than output power consumed by load, the DC-link voltage may decrease.
The DC-link capacitor 122 may solve instantaneous power unbalance of input/output (I/O) terminals. The inverter unit 123 may synthesize an output voltage on the basis of the DC-link voltage, and may include a plurality of power switches. The cell controller 124 may be independently allocated to each unit power cell 120, and may provide a gating signal for deciding a power switching state of the inverter unit 123.
FIG. 3 is a block diagram illustrating a conventional voltage/frequency (V/F) control system.
Referring to FIG. 3, if a user inputs a reference frequency (ωref), a reference voltage generation unit may generate the magnitude (VV/f) and frequency (ωV/f) of a reference voltage generated from the inverter 100 in response to the corresponding reference frequency. The inverter 100 may synthesize 3-phase pulse width modulation (PWM) voltages (Vas, Vbs, Vcs) corresponding to the reference voltage, thereby driving the motor 300.
FIG. 4 is a graph illustrating the relationship between an output frequency and an output voltage of the inverter for use in the voltage/frequency (V/F) control system.
Referring to FIG. 4, since the output frequency (ωref) starts from 0 Hz during initial starting, the inverter outputs a low voltage. As the frequency gradually increases, the inverter outputs a voltage having an amplitude proportional to the increasing frequency. If the output frequency reaches a target frequency, the voltage/frequency (V/F) control system does not increase the frequency any longer, and operates at a constant speed.
The voltage/frequency (V/F) control method is a motor control method widely used in various industrial fields. The voltage/frequency (V/F) control method may be advantageous in terms of speed control and ease of implementation. Generally, the voltage/frequency (V/F) control method has been widely used in industrial fields not requiring rapid operation characteristics in an operation region having a rated speed or less. However, since the slip frequency increases during a high-load operation, the motor rotates at a speed different from a user-input speed, resulting in reduction of speed accuracy. That is, the slip frequency caused by load occurs in a variable-load operation condition, resulting in occurrence of a speed error.